Mud rocks, such as mudstone and shale comprise some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.

In 1967, Tepco chopped 25 meters off the 35-meter natural seawall where the reactors were to be located, according to documents filed at the time with Japanese authorities. That little-noticed action was taken to make it easier to ferry equipment to the site and pump seawater to the reactors. It was also seen as an efficient way to build the complex atop the solid base of bedrock needed to better protect the plant from earthquakes.

But the razing of the cliff also placed the reactors five meters below the level of 14- to 15-meter tsunami hitting the plant March 11, triggering a major nuclear disaster resulting in the meltdown of three reactor cores.

A former senior Tepco executive involved in the decision-making says there were two main reasons for removing the cliff. First, a lower escarpment made it easier to deliver heavy equipment used in the plant, such as the reactor vessels, turbines and diesel generators, all of which were transported to the site by sea. Second, the design of the plant required seawater to keep the reactor cool, which was facilitated by a shorter distance to the ocean.